Rocks have come to their present condition as the result of profound changes in physical structure and in the physical and chemical relations of the component minerals. They therefore offer for investigation a number of problems of broad scope, requiring for their solution not only the application of the principles of geology, but also of physics, chemistry, and allied sciences. The problems are also complex in detail and do not admit of satisfactory analysis without correlating a great variety of evidence, both from the field and from the laboratory. The field evidence was, of course, the first to be sought; forces had to be recognized before they could be measured; but, having been identified, the next step is to seek to establish their relation to each other in the laboratory and to make use of the evidence from experiment for further and more accurate field studies.

Until recently, petrologists . . .

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